This patch introduces a cgroup identifier entry field in perf report to
identify or distinguish data of different cgroups. It uses the device
number and inode number of cgroup namespace, included in perf data with
the new PERF_RECORD_NAMESPACES event, as cgroup identifier. With the
assumption that each container is created with it's own cgroup namespace,
this allows assessment/analysis of multiple containers at once.

Shown below is the output of perf report, sorted based on cgroup id, on
a system that was running three containers at the time of perf record
and clearly showing one of the containers' considerable use of kernel
memory in comparison with others:

While this is a start, there is further scope of improving this. For
example, instead of cgroup namespace's device and inode numbers, dev
and inode numbers of some or all namespaces may be used to distinguish
which processes are running in a given container context. Also, scripts
to map device and inode info to containers sounds plausible for better
tracing of containers.